Journal of Petrology Advance Access originally published online on January 28, 2005
Journal of Petrology 2005 46(4):799-827; doi:10.1093/petrology/egi001
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Chronology, Petrology and Isotope Geochemistry of the Erro–Tobbio Peridotites (Ligurian Alps, Italy): Records of Late Palaeozoic Lithospheric Extension
1 DIPARTIMENTO PER LO STUDIO DEL TERRITORIO E DELLE SUE RISORSE, UNIVERSITÀ DI GENOVA, CORSO EUROPA 26, I-16132 GENOVA, ITALY
2 MAX PLANCK INSTITUT FÜR CHEMIE, POSTFACH 3060, D-55020 MAINZ, GERMANY
RECEIVED AUGUST 12, 2003; ACCEPTED NOVEMBER 22, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSTRUCTURAL BACKGROUND AND...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Mantle peridotites from the Erro–Tobbio (ET) ophiolitic unit (Voltri Massif, Ligurian Alps) record a tectono-metamorphic decompressional evolution, indicated by re-equilibration from spinel- to plagioclase- to amphibole-facies conditions, and progressive deformation from granular to tectonite to mylonite fabrics. The peridotites are considered to represent subcontinental lithospheric mantle that was tectonically denuded during rifting and opening of the Jurassic Ligurian Tethys ocean, similar to the Northern Apennine (External Ligurides) ophiolitic peridotites. We performed chemical and isotopic investigations on selected granular and tectonite spinel peridotites and plagioclase tectonites and mylonites, with the aim of defining the nature of the mantle protoliths, and to date the onset of exhumation of the ET peridotites. Spinel- and plagioclase-bearing tectonites and mylonites exhibit heterogeneous bulk-rock major and trace element composition, despite rather homogeneous mineral chemistry, thus indicating that the ET mantle protoliths record a composite history of partial melting and melt migration by reactive porous flow. The lack of correlation between the observed geochemical heterogeneity and the structural type (granular, tectonite, mylonite) indicates that the inferred reactive porous flow event preceded the exhumation-related lithospheric history of the Erro–Tobbio mantle. The tectono-metamorphic evolution caused systematic chemical changes in minerals: (1) Al decrease in orthopyroxene; (2) Al decrease, and Cr and Ti increase in spinels; (3) Al and Sr decrease, Cr, Ti, Zr, Sc, V and middle to heavy rare earth element increase and development of a negative Eu anomaly in clinopyroxene. The studied samples have Nd isotope compositions consistent with a mid-ocean ridge basalt mantle reservoir. Sm/Nd isotope data on plagioclase and clinopyroxene separates (and corresponding whole rocks) from two plagioclase peridotites, representative of the plagioclase-bearing mylonitic extensional shear zone, have yielded ages of 273 ± 16 Ma and 313 ± 16 Ma, for the plagioclase-facies recrystallization stage, significantly older than the expected Jurassic age. This indicates that the Erro–Tobbio peridotites represent subcontinental lithospheric mantle that was tectonically exhumed from spinel-facies depths to shallower lithospheric levels during Late Carboniferous–Permian times. Our results are consistent with the previously documented evidence for an extensional regime in the Europe–Adria lithosphere during Late Palaeozoic time, and they represent the first record that extensional mechanisms were also active at lithospheric mantle levels.
KEY WORDS: plagioclase-bearing peridotites; subcontinental lithospheric mantle; mantle exhumation; Sm/Nd dating
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSTRUCTURAL BACKGROUND AND...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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The Alpine–Apennine ophiolites represent lithosphere remnants of the Ligurian Tethys ocean, generated by passive rifting, thinning and breakup of the Europe–Adria continental lithosphere connected to the Triassic–Jurassic opening of the Central Atlantic. Previous petrological and isotopic studies on the Northern Apennine (NA) ophiolites suggest that they do not represent mature oceanic lithosphere formed at a mid-ocean ridge setting. In particular, none of the peridotites represent residual asthenospheric mantle that melted and produced mid-ocean ridge basalt (MORB) magmas during Jurassic times. Instead, the NA ophiolites consist of older (Proterozoic and Permian) subcontinental lithospheric mantle, which was intruded by younger (Jurassic) MORB-type magmas (Piccardo et al., 1994
; Rampone et al., 1995, 1998; Rampone & Piccardo, 2000). The ophiolites thus record the structure and composition of the oceanic lithosphere that floors embryonic oceans developed through passive continental rifting and slow-spreading oceanization (Rampone & Piccardo, 2000).
Peridotites from the Erro–Tobbio (ET) Unit (Voltri Massif, Ligurian Alps) are believed to be the Alpine equivalents to the Ligurian ophiolites, except that the Erro–Tobbio Unit was caught up and partly metamorphosed by the Alpine orogeny (Bezzi & Piccardo, 1971; Hoogerduijn Strating et al., 1990, 1993; Scambelluri et al., 1991). Despite the Alpine overprint, the Erro–Tobbio Unit preserves kilometre-scale volumes of unaltered peridotites that retain mantle textures and mineral assemblages, thus allowing the study of the pre-Alpine mantle evolution (Ernst & Piccardo, 1979; Hoogerduijn Strating et al., 1990, 1993; Piccardo et al., 1990; Vissers et al., 1991). Petrological and structural studies have shown that the Erro–Tobbio peridotites experienced a composite tectono-metamorphic decompressional evolution, along kilometre-scale shear zones. This is recorded by subsolidus re-equilibration from spinel- to plagiocase- to amphibole-facies conditions, and progressive deformation from granular to tectonite to mylonite fabrics (Hoogerduijn Strating et al., 1990, 1993; Vissers et al., 1991). The shear zone structures have been interpreted as fragments of an extensional detachment system. On the basis of these features, it has been inferred (Hoogerduijn Strating et al., 1993) that the Erro–Tobbio peridotites represent subcontinental lithospheric mantle that was tectonically denuded by subsolidus decompression in response to the rifting and opening of the Jurassic Ligurian Tethys ocean, similar to the External Liguride (EL) peridotites (Rampone et al., 1995). However, no time constraints have so far been available for this pre-Alpine evolution. In this paper, we present new isotopic and chemical data that indicate that the recrystallization in the plagioclase facies (during exhumation) of the Erro–Tobbio mantle sector occurred much earlier than the 165 Ma exhumation age of the External Ligurides (Rampone et al., 1995), at about 300 Ma.
The primary compositional features of the ET spinel peridotites, and their related mantle processes, have been discussed in a separate paper (Rampone et al., 2004). A summary of the major results of this work, which focused on spinel peridotite samples only, is reported here and integrated with new data on the plagioclase-bearing tectonites and mylonites. In this study we aim to: (1) present an integrated model for the origin of the ET mantle protoliths; (2) address the subsolidus lithospheric evolution recorded by these peridotites; (3) discuss the geodynamic significance of the Erro–Tobbio mantle in the context of the pre-Alpine evolution of the Alpine belt.
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STRUCTURAL BACKGROUND AND SAMPLING |
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TOPABSTRACTINTRODUCTIONSTRUCTURAL BACKGROUND AND...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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The peridotites investigated here were collected from the northeastern part of the Erro–Tobbio Unit (see Fig. 1), in the same area in which Hoogerduijn Strating et al. (1993)
have performed previous structural and petrological studies. Peridotite samples have been selected to represent the different pre-Alpine mantle structures documented by Hoogerduijn Strating et al. (1993); they mostly consist of partly serpentinized cpx-poor lherzolites and harzburgites (cpx <10 vol. %).
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According to Hoogerduijn Strating et al. (1993)
, the oldest recognized fabrics consist of coarse-grained (0·5–1 cm) granular spinel peridotites, transected by different generations of shear zone structures that comprise porphyroclastic spinel-bearing tectonites, spinel-, plagioclase-, hornblende- and chlorite-bearing mylonites, and serpentinite mylonites. The investigated granular spinel peridotites (samples ETR1, ETR2, ETR3, ETR4, ET4/3, ET11/27) consist of metre-scale homogeneous and massive peridotites that have been collected within a few metres distance of each other (see Fig. 1 for sample location). They display protogranular to porphyroclastic textures, and consist of large (0·5–1 cm) olivine and orthopyroxene, and smaller (<0·5 cm) clinopyroxene and spinel grains. Spinel occurs both as single larger grains and in (orthopyroxene + spinel) symplectitic aggregates, mostly confined to the rims of large orthopyroxene porphyroclasts. Olivine and pyroxene show little or no optical evidence for internal plastic strain. Orthopyroxene (and clinopyroxene) porphyroclasts frequently display irregular outlines, caused by embayments of olivine.
In the field, the granular peridotites show variable overprinting that grades from low-strain weakly foliated spinel tectonites to high-strain strongly foliated spinel tectonites (samples ETR14, ETR16, ETA10, ETA20). In the low-strain spinel tectonites, olivine grains show undulatory extinction and sharp subgrain boundaries. In the high-strain spinel tectonites, progressive deformation enhanced dynamic recrystallization of the granular assemblage: these rocks display smaller (0·5–1·5 mm) tabular-shaped olivine grains, and elongated porphyroclasts of orthopyroxene and spinel. Clinopyroxene porphyroclasts are rarely preserved, being mostly recrystallized to smaller (0·1–0·5 mm) equiaxial clino- and orthopyroxene grains. Granoblastic aggregates, consisting of olivine + pyroxenes + spinel, crystallize at the expense of pyroxene porphyroclasts. Both the lattice and the grain-shape fabric of olivine, as well as the asymmetric porphyroclast systems defined by recrystallized clinopyroxenes, indicate that the development of the tectonite foliation involved a component of non-coaxial deformation (Hoogerduijn Strating et al., 1993).
In the southern part of the area mapped by Hoogerduijn Strating et al. (1990), the tectonite foliation is overprinted by a mylonitic fabric developed in mylonitic shear zones of decimetre to 100 m scale (see Fig. 1). The mylonitic microstructures are characterized by lenticular domains formed of small clinopyroxene and extremely stretched orthopyroxene porphyroclasts, surrounded by a fine-grained (10–150 µm) recrystallized olivine mosaic. Again, olivine lattice fabrics indicate an important non-coaxial deformation in the mylonite zones. Apart from the mylonitic shear zones transecting earlier tectonite foliation, mylonitic fabrics also occur in some high-strain tectonites as millimetre-scale bands of ultrafine-grained olivine parallel to the tectonite foliation. Samples ETR11 and ETR12 are partially mylonitized high-strain spinel tectonites, characterized by ultrafine-grained olivine bands, surrounding spinel tectonite relics. Samples ETR6, ETR7, ETR9, ETR10, ETR24, ET1/7 are high-strain plagioclase-bearing tectonites. In these samples, plagioclase is almost completely replaced by low-T recrystallization products, thus precluding a detailed study of plagioclase-bearing microstructures. Plagioclase mostly occurs as rims around elongated spinel porphyroclasts, and in the recrystallized aggregates (mostly consisting of new pyroxenes and plagioclase) replacing pyroxene porphyroclasts.
On the basis of the stable mineral assemblage in the mylonitic recrystallization, Hoogerduijn Strating et al. (1993) have distinguished several types of peridotite mylonites, i.e. spinel-, plagioclase-, hornblende- and chlorite-bearing mylonites. In the plagioclase-bearing mylonites (samples ETR13, ETR22, ET10/5), spinel porphyroclasts are transformed into intensely flattened aggregates of dark brown Cr-spinel rimmed by plagioclase (Fig. 2a and c), with a few plagioclase clasts showing internal plastic deformation (undulatory extinction and mechanical twins) dispersed in the fine-grained olivine matrix. This suggests that plagioclase was already stable in the tectonite assemblage. Orthopyroxene porphyroclasts are extremely stretched and deformed, whereas clinopyroxene porphyroclasts are mostly replaced by a fine-grained aggregate of pyroxenes and plagioclase (Fig. 2d–f). The few preserved cpx porphyroclasts frequently show tiny orthopyroxene + plagioclase exsolution blebs (Fig. 2d). Microtextures in the plagioclase-bearing mylonites suggest that the dynamic subsolidus recrystallization of the peridotites into a plagioclase-bearing mantle assemblage occurred in response to mylonitic deformation.
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Subsequent hornblende- and chlorite-bearing mylonitic recrystallization occurred at the expense of both spinel and plagioclase mylonites. In a few samples studied (e.g. ETR7) local amphibole (Mg-hornblende)-bearing recrystallization has led to the development of radial amphibole aggregates replacing pyroxene porphyroclasts. All the peridotite structures described above are cut by gabbroic and basaltic dykes showing MORB affinity (Hoogerduijn Strating et al., 1990
). Previous petrological investigations of the different peridotite lithotypes (Hoogerduijn Strating et al., 1993) have demonstrated a systematic correlation between microstructures and mineral compositions that have been related to changing (progressively decreasing) P–T conditions during the various stages of syntectonic recrystallization in the shear zones.
In summary, on the basis of field, microstructural and petrological evidence, it has been inferred that the shear zone structures in the Erro–Tobbio peridotites represent fragments of an extensional detachment system, which caused tectonic exhumation and shallow exposure of large (kilometre-scale) mantle fragments (Vissers et al., 1991; Hoogerduijn Strating et al., 1993).
Continuing field and petrological investigations both in the area mapped by Hoogerduijn Strating et al. (1993) and in the southernmost sector of the ET peridotite body have revealed evidence of diffuse melt percolation–impregnation, and multiple episodes of MORB-type gabbroic intrusions (Borghini et al., 2004; De Ferrari et al., 2004; Piccardo et al., 2004a, 2004b). At the outcrop scale, melt impregnation is recorded by the occurrence of peridotites significantly enriched in plagioclase, cut by a network of dunite channels and gabbronoritic dykelets. These dykes and channels show cross-cutting relationships with respect to the tectonite and mylonite spinel peridotite foliation, thus suggesting that melt impregnation and dunite formation postdate the spinel-facies lithospheric deformation (Piccardo et al., 2004a, 2004b). In places, plagioclase-enriched peridotites and associated dunites are deformed and show porphyroclastic to tectonite fabrics. In the less deformed plag-rich tectonites, melt impregnation is suggested by peculiar microstructures: (1) occurrence of plagioclase blebs and veins between mantle olivine; (2) crystallization of large undeformed ‘poikilitic’ orthopyroxene crystals, which partly replace tectonitic mantle olivine; (3) partial dissolution of deformed mantle clinopyroxene and substitution by orthopyroxene + plagioclase intergrowths. Similar microstructures have been described previously in the impregnated plagioclase peridotites from the Alpine–Apennine ophiolites (Rampone et al., 1997, 2003; Piccardo et al., 2004a, 2004b, 2004c), and interpreted as evidence that the peridotites, after their incorporation into the lithosphere, were impregnated by opx-saturated melts. Similar microstructures are not observed in the studied plagioclase-bearing high-strain tectonites and mylonites. In these peridotites, the only textural feature that could be eventually related to melt impregnation is the occurrence of a few plagioclase clasts dispersed in the fine-grained mylonitic matrix. These features could represent either magmatic plagioclase crystallized from impregnating melts or metamorphic plagioclase crystallized as rims around spinel and dismembered by the mylonitic foliation. No compositional differences have been observed, in these samples, between plagioclase in the different microtextural domains. Despite the above uncertainty, i.e. whether some magmatic plagioclase was present before the mylonitic extension-related deformation, textural features indicate that the studied plagioclase-bearing peridotites have been almost completely recrystallized dynamically into a plagioclase-facies mantle assemblage, in which plagioclase crystallizes in association with new olivine and pyroxenes.
Plagioclase formation in the ET peridotites was, therefore, probably related to both dynamic subsolidus recrystallization along extensional shear zones during progressive exhumation and impregnation by ascending melts. On the basis of microtextural features and field occurrence, we infer that the plagioclase-bearing peridotites studied here represent the low-pressure (plagioclase facies) re-equilibration and recrystallization of the ET peridotites during their exhumation along kilometre-scale extensional shear zones.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONSTRUCTURAL BACKGROUND AND...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Whole-rock major and trace (Ni, Co, Sc, V, Cr) element compositions of 15 selected samples were determined by X-ray fluorescence (XRF) at the Dipartimento di Scienze della Terra, University of Modena. Bulk rare earth elements (REE), Sr and Zr concentrations were determined by inductively coupled plasma-mass spectrometry (ICP-MS) at the Institut des Sciences de la Terre, de l'Eau et de l'Espace in Montpellier (France). Outlines of the analytical procedures have been reported by Ionov et al. (1992)
. Bulk-rock major and trace element data for samples ET4/3, ET11/27, ETA20, ETA10, ET1/7 and ET10/5 are from Hoogerduijn Strating et al. (1990) and Robbiano (1994), and were obtained by XRF at DIPTERIS, University of Genova, and by ICP-MS at Montpellier.
Mineral major element compositions were determined by: (1) a Philips SEM 515 electron microscope equipped with an energy dispersive analyser (accelerating potential 15 kV; sample current 20 nA) at the Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova; (2) an ARL SEMQ electron microprobe (accelerating potential 15 kV; beam current 15 nA) at the Centro di Studi per la Stratigrafia e la Petrografia delle Alpi Centrali, Dipartimento di Scienze della Terra, University of Milan. In situ trace element mineral analyses were carried out by secondary ionization mass spectrometry (SIMS) and laser ablation microprobe (LAM)-ICP-MS techniques at IGC-CNR (Istituto di Geoscienze e Georisorse) in Pavia. Detailed descriptions of these analytical procedures have been given by Rampone et al. (1993) and Tiepolo et al. (2003). Bulk-rock and mineral trace element abundances have been normalized to the average C1 chondrite composition (Anders & Ebihara, 1982). Representative mineral compositions of samples ET4/3, ET11/27, ETA10 and ETA20 are from Robbiano (1994).
Isotope analyses were performed at the Max Planck-Institut fur Chemie in Mainz by thermal ionization mass spectrometry (TIMS). Most of the analyses were performed on clinopyroxene separates. For each sample, ‘clean’ (carefully handpicked) clinopyroxenes have been analysed. In a few samples, ‘dirty’ clinopyroxene aliquots (directly obtained from the electromagnetic separator) and whole-rock powders have also been analysed. Isotope analyses on ‘dirty’ and ‘clean’ clinopyroxene aliquots yielded identical results (within the analytical uncertainty). In samples ETR22 and ETR13 (significantly recrystallized in plagioclase-facies conditions and containing unaltered plagioclase), both plagioclase and clinopyroxene were separated and analysed to measure Sm–Nd internal isochrons. Different leaching procedures were used for mineral separates and whole-rock powders. Clinopyroxenes and plagioclase separates were mildly leached with 1N HCl in an ultrasonic bath and washed several times with MilliQ-H2O. Both leached and unleached aliquots of whole-rock powders were analysed. Whole-rock powders were severely leached in hot 6N HCl and the residue was thoroughly washed with H2O. Sample weights varied in the range 50–60 mg for carefully handpicked clean clinopyroxenes, 80–90 mg for ‘dirty’ clinopyroxenes, 150 mg for unleached whole rocks, and 300 mg for leached rocks. For each sample, clinopyroxenes (plus plagioclase and whole rock, if available) were spiked with 150Nd/149Sm, to determine Sm and Nd concentrations by isotope dilution (ID) and dissolved in an HF–HNO3 mixture. REE, Sr and Rb were separated using cation exchange columns. Sm and Nd separations were performed on a second cation exchange column using alpha-hydroxy-isobutyric acid (HIBA) (buffered at pH 4·5). Total procedural blanks were 30 pg for Sm and Nd and 150 pg for Sr. Isotope analyses were carried out on a FINNIGAN-MAT 261 multicollector mass spectrometer. Sr was loaded with TaF5 onto single W filaments. Sm was loaded on double Re filaments. Nd isotope compositions were measured using two techniques: (1) Nd metal method using a double filament technique; (2) Nd oxide method using a single filament technique (Abouchami et al., in preparation). Nd measured as metal was diluted in 2·5N HCl and loaded on a Re filament to which a microdrop of H3PO4 was added. The Nd oxide method developed in Mainz consists of loading the sample on a tungsten filament using a TaF5 activator (see Abouchami et al., in preparation). During the period of analyses (October 1998–April 1999) the La Jolla Nd standard and SRM-NBS 987 Sr standards yielded 143Nd/144Nd = 0·511863 ± 34 (13 analyses) with the Nd metal technique, 143Nd/144Nd = 0·511865 ± 37 (27 analyses) with the Nd oxide technique and 87Sr/86Sr = 0·710235 ± 29 (14 analyses). Mass fractionations corrections were made using 146Nd/144Nd = 0·72190 and 86Sr/88Sr = 0·11940.
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RESULTS |
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TOPABSTRACTINTRODUCTIONSTRUCTURAL BACKGROUND AND...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Whole-rock chemistry
Major and trace element compositions of the Erro–Tobbio peridotites are reported in Table 1. Bulk-rock and mineral analyses for eight spinel peridotite samples have been published previously by Rampone et al. (2004)
and are reported here for completeness. The peridotites are partly serpentinized [loss on ignition (LOI) = 5·38–10·96 wt %]. They define rather large compositional ranges (1·01–3·01 wt % Al2O3; 1·04–2·80 wt % CaO) and display inverse correlations between Al2O3, CaO, Sc, V, TiO2 and MgO contents (Fig. 3; Table 1). Such compositions are similar to those of variably depleted peridotites (e.g. the Internal Liguride and abyssal peridotites). The ET peridotites also exhibit positive Ni, Co–MgO, and negative Cr–MgO correlations (see Fig. 3 and Table 1). The spinel peridotites have heterogeneous major and trace element compositions that cover the entire compositional range. Plagioclase peridotites have a more homogeneous and rather fertile bulk-rock chemistry (2·01–3·01 wt % Al2O3; 1·89–2·53 wt % CaO) (Fig. 3, Table 1).
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Consistent with their major element chemistry, the peridotites exhibit light REE (LREE)-depleted patterns, with variable middle REE (MREE) to heavy REE (HREE) concentrations (Fig. 4). Spinel peridotites have strongly fractionated LREE to MREE (CeN/SmN = 0·047–0·15; CeN/YbN = 0·012–0·078) and almost flat HREE patterns. The granular peridotites exhibit a particularly large variation in their HREE contents (0·7–1·9 x chondrite), although maintaining similar LREE fractionation (CeN/YbN = 0·027–0·078). Plagioclase peridotites display more homogeneous REE patterns, with slightly less marked LREE depletion (CeN/SmN = 0·071–0·13; CeN/YbN = 0·039–0·079) and HREE contents at 1–2 x chondrite. As for the spinel peridotites, they do not exhibit any significant EuN anomaly.
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Major and trace element mineral chemistry
Representative major and trace element mineral compositions of the studied peridotites are reported in Tables 2–7. Olivines have a forsteritic composition and display a limited compositional range (Fo = 88·9–90·4) (Table 2). Spinel compositions in the ET peridotites are shown in Fig. 5, together with the compositional fields for spinels from the Northern Apennine (Internal and External Ligurides) ophiolitic peridotites (Rampone et al., 1993
, 1996) and abyssal spinel peridotites (Hellebrand et al., 2002). According to Rampone et al. (1993), the large variation defined by spinels in the External Liguride peridotites results from different extents of metamorphic re-equilibration, from spinel- to plagioclase-bearing assemblages (the highest Cr* values occurring in spinels from plagioclase peridotites), in samples that have a homogeneous bulk-rock chemistry. In contrast, the compositional variation in spinels from abyssal, plagioclase-free, spinel peridotites is a consequence of different degrees of partial melting recorded by the peridotites (up to 18%; Hellebrand et al., 2002). Spinels from the ET spinel peridotites (granular and tectonite types) define a narrow compositional range and plot at the ‘more fertile’ (Al-rich and Cr-poor) end of the abyssal peridotite trend. Spinels from the plagioclase peridotites display much higher Cr and lower Al contents, and are similar to spinels from the EL plagioclase peridotites (see Fig. 5 and Table 4).
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Orthopyroxenes have enstatitic compositions, with Mg values [Mg/(Mg + Fe2+)] mostly in the range 0·89–0·90. Slightly lower Mg values are shown by orthopyroxene in the plagioclase peridotites. In the spinel peridotites, an overall decrease in Al and Cr contents is observed from cores to rims of opx porphyroclasts and to opx in granoblasts and symplectites (see Table 5). In the plagioclase peridotites, orthopyroxenes display an overall decrease in Al contents from porphyroclast cores (2·86–4·79 wt % Al2O3) to porphyroclast rims and granoblasts (1·36–2·50 wt % Al2O3) stable in the plagioclase-facies assemblage.
Plagioclase is mostly replaced by (clino)zoisite + chlorite + calcite ± prehnite aggregates. Unaltered plagioclase only occurs in two samples (ETR13 and ETR22) and displays fairly high An contents in the range 84·8–88·6, consistent with previous published data (Hoogerduijn Strating et al., 1993; Table 3). Plagioclases also have LREE-depleted patterns (CeN/SmN = 0·40–0·41), marked positive Eu anomalies, and very low Sr contents (7·9–9·8 ppm Sr) (Table 7).
Clinopyroxenes in the spinel peridotites display rather low TiO2 (0·29–0·42 wt %) and high Cr2O3 (0·91–1·42 wt %) concentrations (Fig. 6a and b), consistent with the overall depleted signature of the ET peridotites inferred from the bulk-rock chemistry. Clinopyroxene porphyroclasts in the various samples show rather homogeneous major element compositions, despite the observed bulk-rock chemical variability. Slight compositional variations are observed from cores to rims of clinopyroxene porphyroclasts in spinel granular peridotites. In contrast in the spinel tectonites, clinopyroxenes record a systematic Al decrease from porphyroclast cores to porphyroclast rims and granoblastic grains (see Table 6). Clinopyroxenes in the plagioclase tectonites display significant major element compositional zoning, related to the spinel- to plagioclase-facies transition. Such variations are well recorded by samples ETR6, ETR7, ETR9 and ETR10. In these samples, the cores of spinel-facies clinopyroxene porphyroclasts retain high Al contents (5·97–7·41 wt % Al2O3), similar to those of clinopyroxene porphyroclasts in the spinel peridotites (Fig. 6a and b). Rims of porphyroclasts and granoblastic grains in the plagioclase-bearing assemblage exhibit lower Al and higher Cr and Ti contents (see Fig. 6c and d and Table 6). In the plagioclase mylonites ETR13 and ETR22, very few cpx porphyroclasts are preserved (<3% by vol.), and these still retain rather high Al contents (see Table 6). In these samples, however, clinopyroxene mostly occurs in recrystallized plagioclase-bearing granoblastic aggregates (see Fig. 2f) and displays low-Al, high-Ti,Cr compositions (Table 6). In the Mg-hornblende-bearing aggregates, clinopyroxene displays a systematic decrease in the Al, Ti, and Cr concentrations consistent with partitioning of these elements into coexisting amphibole (Table 6). Clinopyroxenes in the spinel peridotites also have rather homogeneous trace element compositions, characterized by LREE depletion (CeN/YbN = 0·03–0·12) and nearly flat MREE–HREE patterns at about 9–12 x chondrite (see Fig. 7a and b). Clinopyroxenes in the plagioclase tectonites exhibit much higher MREE to HREE concentrations (at about 15–25 x chondrite) although preserving LREE-depleted (CeN/SmN = 0·02–0·05) REE spectra (Fig. 7c); also, they tend to develop a negative EuN anomaly (Eu* = 0·94–0·76). As observed for the major element abundances, clinopyroxenes in the high-strain plagioclase tectonites (e.g. sample ETR9) still record trace element compositional variations from porphyroclastic cores to porphyroclastic rims and granoblastic grains. These latter exhibit an overall increase in MREE to HREE, Zr, Sc, Ti and V contents, the most pronounced negative EuN anomalies (Eu* = 0·76–0·87) and a decrease in Sr abundances (Table 7). In contrast, in the plagioclase mylonites ETR13 and ETR22, the few preserved cpx porphyroclasts do not show core–rim trace element compositional zoning, their compositions being similar to those of the granoblastic grains pertaining to the plagioclase assemblage (see Fig. 7c).
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Geothermometry
Equilibration temperatures for the different metamorphic stages (spinel- to plagioclase- to amphibole-facies recrystallization) recorded by the Erro–Tobbio peridotites have been obtained applying different geothermometric methods (Wells, 1977
; Sachtleben & Seck, 1981; Brey & Kohler, 1990; Taylor, 1998). The obtained results (Table 8) are consistent with previous work (Hoogerduijn Strating et al., 1993). Application of the different methods yields mean equilibration temperatures that fall, for each metamorphic stage, in a range of 100°C (see Table 8). The oldest spinel-facies equilibration, determined using the composition of sp-facies porphyroclastic pyroxenes in all the peridotites, yields mean temperatures in the range 990–1090°C. Slightly lower temperatures (960–970°C) have been obtained for the subsequent spinel-facies tectonite recrystallization. Progressively falling temperatures are recorded by pyroxenes pertaining to the plagioclase- and amphibole-bearing granoblastic assemblages, which yield average temperatures in the ranges 910–960°C and 790–880°C, respectively (Table 8).
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Geochronology
The crystallization of plagioclase in mantle peridotites causes strong fractionation of the Sm/Nd ratio between plagioclase and clinopyroxene and this generates well-developed, internal Sm–Nd isochrons. Previous application of this method to the External Ligurides had yielded well-defined internal isochrons of 165 Ma (Rampone et al., 1995
). For the current study, we separated plagioclase and clinopyroxene from the two plagioclase mylonites ETR22 and ETR13 that have undergone significant dynamic recrystallization leading to the development of a plagioclase-bearing mantle assemblage. As already stated, cpx porphyroclasts pertaining to the spinel-facies assemblage are very rarely preserved in these rocks; these clinopyroxenes, although preserving a major element compositional zoning (e.g. higher Al at the cores), display uniform (core–rim) trace element chemistry, analogous to the granoblastic plagioclase-facies clinopyroxenes (see Fig. 7c and Table 7). This could be due to the very low diffusion rate of Al in clinopyroxene (Sautter et al., 1988; Van Orman et al., 2001). In these samples, computed plagioclase/clinopyroxene REE partition coefficients are consistent with available Dplag/cpx values (Kelemen et al., 1997b; Suhr et al., 1998), thus indicating that trace element equilibrium has been attained between these two minerals.
The Sm–Nd data (Table 9) plotted in Fig. 8 show that plagioclase, clinopyroxene and whole rocks define two sub-parallel isochrons with ages of 273 ± 16 Ma and 313 ± 16 Ma, respectively. Although the 2
error estimates of these isochrons do not overlap, the difference in age is probably insignificant, considering the small number of data points. If the two ages are indeed identical, a best estimate of the pooled average will be 293 ± 20 Ma. As stated above, geothermometric investigations have yielded temperatures in the range 910–960°C for the plagioclase-facies recrystallization. Given the rather high closure temperature of the Sm–Nd isotope system (Mezger et al., 1992), the obtained ages can be considered as a reliable estimate of the time of low-pressure (plagioclase-facies) re-equilibration of the ET mantle. Thus, we will consider 293 ± 20 Ma as the best estimate for the onset of exhumation. Subsequently, we will use an approximate age of 300 Ma, considered to represent a minimum age of incorporation in a lithospheric environment of the ET mantle, to calculate initial isotopic compositions in the following section. This age is significantly higher than the ages of the otherwise similar EL plagioclase peridotites (163–165 ± 20 Ma). The regional tectonic significance of this observation will be discussed further below.
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Sr and Nd isotope geochemistry
We analysed Sr and Nd isotope ratios of separated clinopyroxenes from 10 selected peridotites, representative of the entire tectono-metamorphic evolution (Table 9). In four samples, the Nd isotope ratios analysed in both clinopyroxene and whole rock are similar (see also Fig. 8), whereas the Sr isotope ratios are generally higher in the whole rocks than in the corresponding clinopyroxene. In most cases, each sample aliquot has been analysed repeatedly. Values plotted in Fig. 9 are weighted means of Nd and Sr isotope ratios for each sample (see Table 9). The Sm and Nd concentrations in clinopyroxenes obtained by isotope dilution (ID) are generally different (by about 30–40%) from corresponding in situ SIMS and LAM-ICP-MS data. Such differences have previously been documented in peridotites recording various equilibration stages and have been ascribed to inter- and intra-grain inhomogeneity (Vannucci et al., 1994
; Rampone et al., 1996). Despite the observed differences between absolute concentrations, the Sm/Nd ratios in clinopyroxene, derived from the two datasets, are consistent. The Sm and Nd contents in whole rocks analysed by ID and ICP-MS are very similar, varying within 10%.
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Clinopyroxene separates from the ET peridotites display rather homogeneous and depleted present-day Nd isotope ratios (143Nd/144Nd = 0·513209–0·513385), coupled with high 147Sm/144Nd ratios (0·382–0·471). In contrast, the Sr isotope ratios vary significantly (87Sr/86Sr = 0·703019–0·704769). The lack of an Sr–Nd correlation is frequently observed in ophiolites and present-day oceanic lithosphere (e.g. McCulloch et al., 1980
; Rampone et al., 1998) and is probably caused by interaction with seawater-derived fluids that have little effect on Nd isotope ratios, because of their negligible REE contents, but cause an increase in 87Sr/86Sr ratios. In the case of the ET peridotites, this interpretation is confirmed by systematically lower Sr isotope ratios (87Sr/86Sr = 0·703456–0·704938) in leached whole-rock aliquots, compared with the corresponding unleached aliquots (87Sr/86Sr = 0·704534–0·705909; see Table 9). Nevertheless, clinopyroxene separates that have been carefully handpicked and leached still exhibit high Sr isotope ratios. In one sample (ETR12), the leached whole-rock aliquot actually shows a lower 87Sr/86Sr ratio (0·703514) than the corresponding clinopyroxene (0·704769). This could be due to the occurrence, in the cpx separates, of small amounts of tremolitic amphibole and/or serpentine developed at the expense of clinopyroxene and/or opx exsolution during low-grade alteration. A similar interpretation has been proposed to explain anomalously high Sr isotope ratios in separated clinopyroxenes from the Northern Apennine ophiolitic gabbroic sequence (Rampone et al., 1998). In the plagioclase peridotites, plagioclase has slightly higher Sr isotope ratios (0·704331 and 0·704934) than the corresponding clinopyroxene (0·704237 and 0·704648, respectively). Again, such high Sr isotope ratios are probably caused by plagioclase saussuritization during low-grade oceanic alteration.
To compare the isotopic compositions of the ET peridotites with those from other peridotite bodies and with modern mantle-derived basalts (see Fig. 9), we have corrected the data in two stages. First, we calculate the initial Nd isotope ratios to the inferred age of melting and/or emplacement of the peridotites using the measured 147Sm/144Nd ratios. Then we recalculate the initial Nd isotope ratios to the present assuming a 147Sm/144Nd ratio of 0·222 [Depleted Mantle (DM) source]. For the Erro–Tobbio peridotites, we used an emplacement age of 300 Ma (see above). For the Internal Liguride and Lanzo we used, respectively, 275 Ma (inferred age of depletion, Rampone et al., 1996) and 200 Ma (assumed age of emplacement, Bodinier et al., 1991). The EL peridotites do not need any Nd decay correction because their 147Sm/144Nd ratios (0·23–0·25) are close to present-day MORB mantle. We applied no age correction to the Sr isotope ratios because of the secondary alteration problems mentioned above.
In Fig. 9, it is evident that all the peridotite Nd isotope data, after the decay correction, are similar to modern MORB. Most of the peridotite massifs (Northern Apennines, Lanzo North) plot at the depleted end of the MORB field. A few samples from the External Ligurides and Lanzo North, particularly, have extremely low Sr isotope ratios (coupled with very high Nd isotope ratios), which have been interpreted to reflect a long time of isolation from the convecting asthenosphere and incorporation in the subcontinental lithosphere (Bodinier et al., 1991; Rampone et al., 1995). The corrected Nd isotope ratios of the Erro–Tobbio peridotites extend towards slightly more enriched values (0·512729–0·513013). Such ratios are still consistent with a MORB mantle reservoir, although this inference is weakened by the uncertainty of the primary Sr isotope compositions (see above). No correlation is recognizable between the Nd isotope signature of the peridotites and their textural-paragenetic type (spinel granular vs plagioclase tectonite peridotites; see Table 10). Similarly, no correlation exists between the Nd isotope ratios and the corresponding bulk-rock REE compositions. For example, the two samples with similarly depleted REE bulk-rock signature (spinel tectonite ETR16 and spinel granular peridotite ETR4) have, respectively, the most ‘enriched’ (0·512729) and the most depleted (0·512972) recalculated Nd isotope ratio. Overall, both the Erro–Tobbio and the Lanzo peridotites have very similar (age-corrected) Nd isotope compositions, which cover most of the compositional range defined by present-day MORB (Hofmann, 1997).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSTRUCTURAL BACKGROUND AND...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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The Erro–Tobbio mantle protoliths: evidence for reactive porous flow
The major and trace element bulk-rock compositions of the Erro–Tobbio peridotites record a variably depleted signature. In principle, these features could be interpreted as reflecting mantle residua after removal of various amounts of partial melt. However, the comparison between bulk-rock and mineral compositions indicates that these rocks cannot be simple residua after melt extraction. Rather, they probably record a composite history of partial melting and melt migration by reactive porous flow. These aspects have been discussed in detail by Rampone et al. (2004)
, in a study specifically focused on a selected set of spinel peridotites. In this section, we summarize the major lines of evidence supporting this interpretation and integrate the discussion on the ET mantle protoliths considering new chemical and isotopic data on the spinel and plagioclase peridotites.
Minerals in the spinel peridotites (both granular and tectonite types) display rather uniform major (and trace) element compositions (see Figs 5–7). In spite of this, the bulk-rock compositions define large ranges and striking correlations, i.e. increasing FeOtot, Ni and Co, and decreasing Al2O3, SiO2, CaO, Sc, Cr and YbN, with increasing MgO. The MgO–FeOtot and MgO–SiO2 correlations (see Rampone et al., 2004) are similar to those recently recognized by Niu et al. (1997) and Asimow (1999) in abyssal peridotites. In those studies, it was emphasized that such correlations are not consistent with various inferred partial melting trends. Rather, they can be explained by combined histories of partial melting and subsequent melt migration, which caused either olivine addition (Niu et al., 1997) or dissolution–precipitation reaction (Asimow, 1999) by equilibrium porous flow.
As stated above, the ET spinel peridotites show clear evidence that the above trends cannot be interpreted as resulting from progressive depletion, because the observed bulk-rock chemical variability is not mirrored by consistent mineral chemistry variations. This is particularly evident in terms of REE contents. In mantle peridotites resulting from progressive equilibrium (fractional or batch) partial melting, whole-rock and constituent minerals should indicate consistent degrees of depletion, i.e. progressive decrease in HREE absolute concentrations coupled with decreasing LREE/HREE ratios. As shown in Fig. 10a and b, the ET spinel peridotites do not show such consistency. Both bulk-rock and clinopyroxenes display a rather small variation of the CeN/YbN ratio, despite a large range in the bulk-rock YbN contents. The granular and the tectonite–mylonite spinel peridotites define a large and overlapping range in bulk-rock HREE, thus indicating that their bulk-rock heterogeneity is not correlated to the textural type. Bulk-rock chemical variations in the ET spinel peridotites are coupled to systematic modal changes, i.e. progressive clinopyroxene and orthopyroxene decrease, and olivine increase, with increasing bulk MgO (see Table 1). Rampone et al. (2004) have outlined that the observed modal abundances, particularly the high olivine modal proportions in most spinel peridotites (75–86%), are inconsistent with those predicted in residual peridotites by standard melting models (Niu et al., 1997; Godard et al., 2000). Also, the bulk Cr and YbN decrease, and Ni increase, correlated with modal cpx decrease (Fig. 11a) and modal olivine increase, are consistent with the expected chemical and modal effects, during a process of interaction between depleted peridotites and melts migrating by equilibrium porous flow (involving pyroxene dissolution and olivine precipitation reactions; Godard et al., 1995). This interpretation is further supported by a theoretical study of Mg–Fe partitioning during mantle melting and melt–rock interaction (Bedini et al., 2002), which has shown that the bulk-rock Mg-number vs modal olivine relationship in residual mantle peridotites may be largely controlled by melt–rock reactions. In particular, olivine-forming reactions (involving precipitation of olivine at the expense of orthopyroxene) produce ‘reactive’ harzburgites and dunites, with lower Mg-numbers than predicted by melting models (see Fig. 11b). In contrast, refertilization resulting from pyroxene-forming reactions at decreasing melt mass produces lherzolites with higher Mg-numbers than expected by melting trends (Bedini et al., 2002). As is evident in Fig. 11b, most of the ET spinel peridotites define a trend of nearly constant bulk-rock Mg-number (mostly in the range 0·90–0·905) with increasing modal olivine, consistent with an olivine-forming reaction model. According to this model, spinel peridotites showing the most depleted bulk and modal compositions (e.g. samples ETR4 and ETR16) represent the rocks that have experienced larger extents of melt–rock reaction, i.e. the ‘most affected’ by the reactive porous flow process. Some textural features in the granular peridotites, e.g. the diffuse occurrence of olivine grains causing embayments around orthopyroxene (and clinopyroxene) porphyroclasts, are consistent with the inferred melt–rock reaction causing olivine precipitation and pyroxene dissolution (Piccardo et al., 2004b). On the basis of the above chemical features, Rampone et al. (2004) have argued that the ET spinel peridotites record a composite history of partial melting and melt–rock interaction, as invoked for other ophiolitic and abyssal peridotites (Kelemen et al., 1997a; Asimow, 1999; Godard et al., 2000; Dijkstra et al., 2001; Piccardo et al., 2004a, 2004b, 2004c).
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We now discuss the significance of the studied plagioclase peridotites, in the context of the proposed reactive porous flow history. The plagioclase peridotites display bulk-rock abundances that plot at the ‘more fertile’ end of all trends discussed above (e.g. higher Ca, Al, Sc, V, HREE and modal cpx contents; see Figs 3 and 4). On these grounds, it could be argued that the plagioclase peridotites indeed represent mantle rocks that were refertilized by impregnating melts causing plagioclase and clinopyroxene precipitation, rather than spinel peridotites recrystallized at plagioclase-facies conditions in response to decreasing pressure. As stated in the ‘Structural Background’ section, microtextural features of the studied plagioclase peridotites indicate that the development of a plagioclase-bearing mantle assemblage was related to a dynamic recrystallization event along tectonitic to mylonitic extensional shear zones. Characteristic microstructures such as (1) the crystallization of plag + px + ol granoblastic aggregates at the expense of pyroxene porphyroclasts, and (2) the development of tiny plag + opx exsolution lamellae or blebs in cpx porphyroclasts, are consistent with the subsolidus recrystallization of former spinel peridotites to a new, plagioclase-bearing, mantle assemblage (Rampone et al., 1993
). On a microtextural basis, we therefore infer that the studied plagioclase-bearing ultramafics represent subsolidus re-equilibrated peridotites, rather than former harzburgites refertilized by plag + cpx crystallization. This latter interpretation has been recently proposed for plagioclase peridotites from the Othris ophiolites (Dijkstra et al., 2001; Barth et al., 2003) and the Lanzo massif (Piccardo et al., 2004a). In the Othris peridotites, plagioclase (± ortho- and clinopyroxenes) occurs in veins that preserve a magmatic texture (Dijkstra et al., 2001). In contrast to the Erro–Tobbio peridotites, the Othris mantle ultramafics do not show any petrographic evidence of metamorphic plagioclase-facies recrystallization (e.g. development of plag + opx exsolutions in clinopyroxene), even in fine-grained mylonitic shear zones. Thus, it has been inferred that plagioclase crystallization in the Othris peridotites does not represent a subsolidus exhumation; rather, it resulted from melt impregnation and refertilization of previous harzburgites, which occurred at spinel-facies conditions (Dijkstra et al., 2001; Barth et al., 2003). If plagioclase does indeed represent a subsolidus product in the investigated ET plagioclase peridotites, we must then place their chemical features in the context of the reactive porous flow event recorded by the spinel peridotite protoliths. As noted above, plagioclase-bearing peridotites mostly display rather fertile bulk-rock chemistry. Nevertheless, similar ‘rather fertile’ compositions are shown by some spinel peridotite samples, thus indicating that there is no systematic correlation between bulk-rock chemistry and textural type. Furthermore, the compositions of spinel-facies minerals (pyroxenes and olivine) in the plagioclase peridotites are rather homogeneous, and similar to those of minerals in the spinel peridotites. Relict spinel-facies clinopyroxene porphyroclasts in less deformed samples (high-strain plagioclase tectonites) display major element abundances similar to those of clinopyroxenes in the spinel peridotites (see Fig. 6a and b). In addition, clinopyroxenes analysed in a few plagioclase-bearing peridotites, although having higher overall REE abundances (see discussion below) exhibit an LREE-depleted signature similar to clinopyroxenes in the spinel peridotites. This is highlighted in Fig. 10a and b, where the plagioclase peridotites show no correlation between the CeN/YbN ratio (in bulk rock and cpx) and the bulk-rock YbN, as observed for the spinel peridotites.
Another remarkable feature is shown by the bulk-rock Mg-number vs modal olivine correlation (see Fig. 11b). Two important observations can be made: (1) the plagioclase peridotites, when considering the recalculated modes of their spinel facies precursors, plot at the beginning of the melt–rock reaction trend defined by the spinel peridotites; (2) the estimated modal abundances obtained considering the compositions of plagioclase-facies minerals display higher modal olivine relative to the corresponding spinel-bearing modes. This is consistent with the subsolidus reaction governing the spinel- to plagioclase-facies transition (see discussion below), which implies crystallization of olivine + plagioclase at the expense of pyroxene + spinel. In contrast, as inferred by Bedini et al. (2002), different trends are expected for refertilizing reactions (leading to pyroxene crystallization), which cause significant decrease of the modal olivine contents, at a given bulk-rock Mg-number (see Fig. 11b).
On the basis of the above observations, we speculate that the more ‘lherzolitic’ bulk-rock composition of the plagioclase peridotites resulted from a lower extent of melt–rock reaction affecting their precursor spinel-facies assemblage, rather than from melt impregnation. The evidence that plagioclase-facies recrystallization is mostly recorded by peridotites showing less depleted bulk-rock chemistry could be explained by the fact that Ca- and Al-richer bulk compositions enhanced crystallization of the plagioclase-bearing assemblage. The studied plagioclase peridotites may thus represent, similarly to some ‘less depleted’ spinel peridotites, the ET mantle protoliths that were least affected by the reactive porous flow event. The LREE-depleted signature displayed by clinopyroxene in all the peridotites suggests that the interacting melts had a MORB-type affinity. Geochemical variations caused by melt–rock interaction have been also recognized in the Ronda peridotites (Van der Wal & Bodinier, 1996), and it has been demonstrated that they are correlated with the structural facies; namely, with the development of granular textures at the expense of pristine deformed spinel tectonites. In the Erro–Tobbio peridotites, the systematic bulk-rock compositional variations related to melt–rock reaction are shown both by undeformed (granular) and deformed rock types. As inferred by Rampone et al. (2004), this indicates that the geochemical variability recorded by the bulk rocks, probably caused by a melt–rock interaction process, was acquired before the lithospheric tectono-metamorphic evolution. Thus, the reactive porous flow event probably occurred prior to and/or concomitantly with the incorporation of the ET mantle at deep (spinel-facies) lithospheric levels.
The Erro–Tobbio body shows large areas where the peridotites display evidence of diffuse melt impregnation, mostly indicated by significant plagioclase enrichment and widespread networks of replacive dunites and gabbronoritic dykelets that cross-cut the tectonite and mylonite foliation. This suggests that the melt impregnation event occurred after the spinel-facies tectonite–mylonite lithospheric deformation, presumably when the peridotites were already exhumed at rather shallow lithospheric levels (in the metamorphic plagioclase stability field; Borghini et al., 2004; Piccardo et al., 2004a, 2004b). The melt impregnation event was, therefore, probably subsequent to, and not directly related to, the reactive porous flow process that affected the spinel-facies ET mantle protoliths.
The Nd isotope composition of the Erro–Tobbio peridotites is similar to that of other regional peridotites such as Lanzo and it is indistinguishable from that of modern depleted to slightly enriched MORB-source reservoirs. Such compositions are also common in the subcontinental lithosphere and they cannot be used to distinguish between particular environments such as oceanic vs subcontinental mantle or plume sources (Bodinier et al., 1991). The Sr isotope ratios of the ET peridotites have probably been modified by seawater alteration and are, therefore, not representative of a primary mantle isotope signature. We have stated above that peridotites showing the least depleted bulk-rock composition (e.g. the plagioclase peridotites and spinel tectonites ETR11 and ETR12) presumably represent rocks that were least affected by the reactive porous flow event, whereas the most depleted peridotites (e.g. spinel tectonite ETR16 and spinel granular peridotite ETR4) record higher extents of melt–rock interaction. As shown in Fig. 9, no simple correlation can be established between the measured Nd isotope composition of the different samples and the extent of melt–rock interaction recorded by their bulk-rock chemistry. Age-corrected Nd isotope ratios in the plagioclase peridotites are almost identical to those of the three analysed granular peridotites (samples ETR1, ETR3 and ETR4). In addition, the two peridotites showing the lowest, slightly more enriched, Nd isotope ratios are the spinel tectonites ETR16 and ETR12, characterized by very different bulk-rock chemistry. This means that we cannot simply relate the observed modest Nd isotope heterogeneity of the ET mantle peridotites to the reactive porous flow event alone. It is noteworthy, however, that the inferred MORB-type affinity of percolating melts is consistent with the overall MORB-type Nd isotope signature of the ET mantle.
The exhumation of the ET mantle
After their incorporation into a lithospheric environment (as shown by static recrystallization in the spinel-facies at T <1100°C), the ET peridotites experienced a composite subsolidus decompressional evolution, involving partial recrystallization from spinel- to plagioclase- to amphibole-facies conditions and progressive deformation (from granular, to tectonite to mylonite fabrics) along kilometre-scale lithospheric shear zones. This metamorphic evolution is documented by major and trace element compositional variations in minerals (especially pyroxenes) recording the different assemblages. As shown by Hoogerduijn Strating et al. (1993) and in the present study, the cores of sp-facies pyroxene porphyroclasts preserve similar compositions in all lithotypes and yield similar equilibration temperatures (990–1090°C). This suggests that the granular protolith attained complete chemical equilibrium before the development of the tectonite and mylonite structures (see Fig. 6 and Table 8).
The subsequent plagioclase-facies recrystallization is recorded by systematic major and trace element compositional variations in the constituent minerals: (1) Al decrease in orthopyroxene; (2) Al decrease, and Cr and Ti increase in spinels; (3) Al and Sr decrease, Cr, Ti, Zr, Sc, V, and MREE to HREE increase and development of a negative Eu anomaly in clinopyroxene (see Figs 5, 6 and 7c). Major and trace element compositional zoning in spinel-facies porphyroclastic pyroxenes is mostly preserved in the plagioclase tectonites (e.g. sample ETR9), whereas in the plagioclase mylonites (e.g. samples ETR13 and ETR22), as a result of intense deformation and dynamic recrystallization, very few clinopyroxene porphyroclasts are preserved. These surviving porphyroclasts do not show trace element compositional zoning and both porphyroclasts and granoblasts display similar (MREE–HREE, Sc, V, Ti, Cr, Zr enriched and Al, Sr depleted) compositions (see Fig. 7c).
Similar chemical changes in minerals have been documented in some plagioclase-re-equilibrated lherzolites from the External Liguride (Northern Apennine) ophiolitic units and the Galicia passive margin (Western Spain), and it has been inferred that they resulted from interphase element partitioning in response to the subsolidus reaction px1 + Al-sp1 
pl2 + ol2 ± px2 + Cr-sp2 (Kornprobst & Tabit, 1988; Rampone et al., 1993). Specifically, Rampone et al. (1993) have shown, using mass balance calculations, that the observed trace element interphase redistribution can result from the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. Rampone et al. (1993) further inferred that the increase in Cr and Ti in spinel relics (sp2) and plagioclase-facies clinopyroxene (cpx2) occurs because these elements cannot be partitioned into the other mineral phases (olivine and plagioclase). In addition, plagioclase can preferentially host Al, Sr and Eu whereas it has very low affinity for Zr, Y, V, Sc and MREE to HREE, whose contents consequently increase in granoblastic plagioclase-facies clinopyroxenes (cpx2) (Rampone et al., 1993). Thus, the compositional variations observed in minerals from the studied ET plagioclase peridotites are consistent with those expected during the subsolidus spinel- to plagioclase-facies transition. The chemical changes recorded by clinopyroxene in the plagioclase mylonites also indicate the attainment of chemical equilibrium with plagioclase. This is supported by the consistency between the patterns defined by plagioclase/clinopyroxene REE concentration ratios in samples ETR13 and ETR22 and available knowledge on REE plag/cpx partitioning (Rampone et al., 1993; Kelemen et al., 1997b; Suhr et al., 1998).
It is remarkable, however, that similar chemical changes in clinopyroxene (i.e. Al, Sr decrease and Ti, Zr, Y, Sc, and MREE to HREE increase) have been documented in the plagioclase-rich impregnated peridotites from Mt. Maggiore (Alpine Corsica, France), where opx-saturated depleted melts are inferred to have caused plagioclase + orthopyroxene crystallization at the expense of mantle clinopyroxene (Rampone et al., 1997; Piccardo et al., 2004a, 2004b). Thus, the origin of plagioclase in mantle peridotites (subsolidus reaction vs melt impregnation product) cannot be easily discerned on the basis of clinopyroxene chemistry, especially when the impregnating melts have a depleted trace element signature (Rampone et al., 1997). However, preliminary mineral chemistry investigations (Borghini et al., 2004) have revealed that spinels in the plagioclase-rich impregnated ET peridotites display similar Cr values (Cr-number = 0·46–0·49) but higher TiO2 contents (in the range 0·76–0·94 wt %), relative to spinels in the plagioclase-bearing peridotites studied here.
Subsequent partial reaction at amphibole-facies conditions is documented by the crystallization, in some spinel and plagioclase tectonites, of a hornblende-bearing granoblastic assemblage. Thermometric estimates on pyroxenes pertaining to the plagioclase-bearing and amphibole-bearing assemblages are in the range 910–960°C and 790–880°C respectively (see Table 8). This indicates that the spinel- to plagioclase- to amphibole-facies decompressional evolution occurred in the subsolidus region, in a lithospheric environment, and was accompanied by progressive temperature fall (see also Hoogerduijn Strating et al., 1993). The late stage of the ET mantle tectonic exhumation is documented by the development of chlorite-bearing and serpentinite mylonites (Hoogerduijn Strating et al., 1993; Scambelluri et al., 1997). As stated by Hoogerduijn Strating et al. (1993), the overall P–T path recorded by the ET peridotites is consistent with the thermal history expected for the footwall of a lithosphere-scale extensional shear zone.
Sm–Nd radiometric investigations performed on the two plagioclase mylonites ETR13 and ETR22, which record the dynamic low-pressure recrystallization of the ET peridotites in response to exhumation along mylonitic extensional shear zones, have yielded an approximate age of 300 Ma for this stage (Fig. 8). This indicates that the onset of exhumation of the ET mantle occurred at a much earlier time than the inferred Triassic–Jurassic rifting and opening of the Ligurian Tethys ocean. A middle Jurassic age (165 Ma) has been obtained for the plagioclase-facies re-equilibration stage in the External Liguride ophiolitic peridotites (Northern Apennines), using the same isotopic methods (Rampone et al., 1995). Thus, the ET and EL peridotites, despite an otherwise similar petrological and tectonic decompressional evolution, record different ages for the same initial stage of exhumation. We speculate that the EL and ET peridotites represented different lithospheric mantle regions, which were activated at different times during extension of the Europe–Adria lithosphere. However, recent isotopic investigations on some eastern EL peridotite bodies have provided older (Triassic) ages for the plagioclase-facies subsolidus recrystallization stage (Montanini et al., 2004). Below, we discuss how these results fit with present knowledge of the pre-Alpine evolution of the Europe–Adria lithosphere.
Inferences on the geodynamic significance of the Alpine–Apennine ophiolites
Previous Nd isotope studies have shown that the Internal Liguride (Northern Apennine) ophiolitic peridotites represent MORB-type asthenospheric mantle that underwent passive upwelling and melting during Permian time. This asthenospheric material was then accreted to the subcontinental lithosphere, and finally exhumed and intruded by Middle Jurassic gabbros and basalts (Rampone et al., 1996, 1998). The Permian time of depletion was inferred by Sm–Nd DM model ages resulting from extremely high Nd isotope ratios (143Nd/144Nd = 0·513644–0·513780) in clinopyroxene from the Internal Ligurides peridotites (Rampone et al., 1996). Permian model ages of depletion have been also documented in mantle peridotites from the Platta Nappe (eastern Swiss Alps), which is considered to represent an ‘ocean–continent transition’; that is, the preserved remnants of the southeastern margin of the Ligurian Tethys (Müntener et al., 2004). The results of this study, together with previous knowledge on the Alpine–Apennine ophiolitic peridotites, thus suggest some similarities, but also considerable diversity, of origin, composition and age of the mantle peridotites that constituted the lithospheric mantle flooring the Ligurian Tethys Ocean. As outlined above, a remarkable feature of the Alpine–Apennine ophiolitic peridotites is the occurrence of preserved records of Permian events, despite their later involvement in the Jurassic extension (and association with Jurassic MORB-type crust). In particular, the Late Carboniferous–Permian age of plagioclase-facies recrystallization in the ET peridotites (obtained by Sm/Nd plag–cpx internal isochrons), and the Permian age of depletion recorded by the Internal Liguride (Northern Apennine) and Platta (Swiss Alps) peridotites (inferred by Sm/Nd model ages on clinopyroxenes), indicate that extension of the Europe–Adria lithosphere and passive asthenosphere upwelling and melting were already active in Permian times.
In the Alpine–Apennine belt, several lines of evidence indicate that, during Permian time, the Europe–Adria lithosphere was dominated by an extensional tectonic regime, and by concomitant asthenosphere upwelling and melting causing intense igneous activity (Diella et al., 1992; Dal Piaz, 1993). Lithosphere extension is primarily indicated by the pre-Alpine extensional retrograde evolution recorded in the South Alpine (Ivrea Verbano) and Austroalpine basement rocks (Diella et al., 1992; Dal Piaz, 1993; and quoted references). Asthenosphere melting and magmatic underplating are largely documented by the emplacement at various crustal levels of large gabbroic complexes (Voshage et al., 1990; Quick et al., 1992; Thoni & Jagoutz, 1992; Tribuzio et al., 1999; Vavra et al., 1999; Hansmann et al., 2001), which record Late Carboniferous–Permian ages of magmatic crystallization and a tholeiitic affinity of the parental melts. Many of these gabbros, although being affected by variable degrees of crustal contamination, preserve Nd–Sr isotope compositions close to that of the depleted mantle, thus testifying to the upwelling and melting of MORB-type asthenospheric mantle (Voshage et al., 1990; Thoni & Jagoutz, 1992; Tribuzio et al., 1999; Montanini & Tribuzio, 2001).
Currently the overall geodynamic significance of the Late Carboniferous–Permian extensional stage is in debate. According to some workers, extensional driving forces continuously operated from the Permian to the Jurassic (Norian) continental rifting and the late Jurassic opening of the Ligurian Tethys (Lardeaux & Spalla, 1991; Diella et al., 1992; Dal Piaz, 1993). In contrast, other workers have related the Permian extensional and magmatic events to post-Variscan transtensional movements (Handy & Zingg, 1991; Bertotti et al., 1993) and argued that Permian extension and associated magmatic underplating and Tethyan rifting represent two distinct geological events (Costa & Rey, 1995; Hermann & Müntener, 1996; Manatschal & Bernoulli, 1999; Müntener et al., 2000). Major evidence supporting the latter hypothesis is the absence of continuous magmatic activity from Permian to Jurassic times. Most ages for gabbroic rocks of the Alpine–Apennine system cluster at 300–250 Ma and 165–150 Ma (Rampone & Piccardo, 2000, and references therein). It is remarkable, however, that continuing isotope studies on gabbroic rocks from the Alpine–Apennine belt are increasingly giving ages in the range 180–240 Ma (e.g. Finero, Peressini et al., 2004; Baldissero, Mazzucchelli et al., 2004; External Liguride ophiolites, Tribuzio et al., 2004), thus providing evidence for magmatic activity from the Permian to the Jurassic. An age of about 180 Ma has also been obtained by preliminary Sm–Nd isotope investigations on gabbroic rocks intruded in the southern sector of the Erro–Tobbio peridotites (work in progress).
The results of this study provide further evidence that the Europe–Adria lithosphere was dominated by an extensional tectonic regime during the Late Palaeozoic and show that extension was also active at deep lithospheric mantle levels. The overall results of Nd–Sr isotope studies on the Alpine–Apennine ophiolitic peridotites, although not solving the question of whether lithosphere extension acted continuously between Permian and Jurassic times, thus provide striking evidence that lithospheric mantle sectors extending and asthenosphere upwelling during the Permian were later involved in the Triassic–Jurassic extension that led to the oceanization of the Jurassic Ligurian Tethys.
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CONCLUDING REMARKS |
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TOPABSTRACTINTRODUCTIONSTRUCTURAL BACKGROUND AND...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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The results of this work provide further evidence for the complex and diverse origin of the Alpine–Apennine ophiolites, which represent an association of tectonically denuded subcontinental lithospheric mantle and younger MORB-type magmatism. This lithological association is expected to develop during the early stages of formation of an embryonic ocean, by passive lithosphere extension (Rampone & Piccardo, 2000
; Whitmarsh et al., 2001). The major steps in the evolution of the ET mantle peridotites can be summarized as follows.
- Pre-Palaeozoic melting and interaction with MORB-type melts, which migrated through the peridotites by reactive porous flow, caused the modal variability (from cpx-poor lherzolites to harzburgites) and contrasting bulk-rock and mineral chemistry recorded by the mantle protoliths. This latter stage (melt–rock interaction) occurred prior to, and/or concomitantly with, the incorporation of the ET mantle in a lithospheric environment (documented by the annealing recrystallization in the spinel facies stability field, at T <1100°C).
- Late Palaeozoic onset of exhumation from spinel-facies mantle depths to shallower lithospheric levels is indicated by partial recrystallization at plagioclase-facies conditions, mostly developed along kilometre-scale extensional shear zones.
- Diffuse melt impregnation caused significant plagioclase enrichment in the peridotites and the occurrence of dunite channels and gabbronoritic dykelets cross-cutting the tectonite and mylonite spinel-facies foliation (Borghini et al., 2004; Piccardo et al., 2004a, 2004b). This event presumably occurred when the peridotites were already exhumed to shallow lithospheric levels (i.e. at plagioclase-facies conditions; Piccardo et al., 2004a, 2004b).
- Multiple episodes of MORB-type melt intrusion at moderate depths (P = 3–5 kbar), are documented by the occurrence of intrusive olivine cumulate pods and olivine gabbro dykes, cross-cutting all previous mantle structures (Borghini et al., 2004; De Ferrari et al., 2004).
- Progressive exhumation is documented by the development of amphibole- and chlorite-bearing peridotite mylonites, and serpentinite mylonites (Hoogerduijn Strating et al., 1993).
- Later (presumably Jurassic) sea-floor exposure is recorded by the intrusion of MORB-type basaltic dykes cross-cutting all above mantle structures.
The subcontinental origin of the Erro–Tobbio mantle peridotites, previously inferred on the basis of their entirely subsolidus decompressional history (Hoogerduijn Strating et al., 1993), is reinforced by the evidence that exhumation of the ET peridotites, in response to lithospheric extension, began much earlier (Late Palaeozoic) relative to the inferred Triassic–Jurassic age of rifting and opening of the Ligurian Tethys ocean. This is consistent with the increasing evidence of Permian extension-related events recorded in many Alpine–Apennine ophiolitic peridotites.
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ACKNOWLEDGEMENTS |
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A. Romairone is indebted to J.-L. Bodinier for the personal and technical assistance during her visit to the Institut des Sciences de la Terre, de l'Eau et de l'Espace in Montpellier, to perform the bulk-rock ICP-MS analyses. M. Mazzucchelli is thanked for having provided the XRF analyses. We also thank A. Zanetti for assistance with the SIMS and LAM-ICP-MS analyses. This work has benefited from discussions with R. Tribuzio, G. Rivalenti, M. Mazzucchelli, G. V. Dal Piaz and M. Scambelluri. G. Suhr, J. Barth, E. Takazawa and G. Davies are acknowledged for constructive reviews. Funding was provided by the Italian MIUR, and the University of Genova.
* Corresponding author. Telephone: 0039 10 3538315. Fax: 0039 10 352169. E-mail: betta{at}dipteris.unige.it
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